Cut potatoes and carrots and add to slow cooker. Then, pour one can Guinness over ingredients, cover and cook on low for 8 hours.
One half hour before corned beef is ready, cut cabbage into wedges and add to large pot of water, bring to boil and cook for 10 minutes; drain cabbage.
In a large deep fry pan, melt butter. Add cabbage to butter and pour your second can of Guinness on top. Cook until most of liquid has cooked off, stirring frequently so as not to burn cabbage.
Once corned beef is cooked, remove from slow cooker and place onto cutting board. The beef should fall apart and not need to be sliced. Then add your cabbage, with the butter/beer liquid to Slow Cooker mixing with potatoes and carrots. Dish out beef into bowls and cover with cabbage potatoes and carrots.